Static and dynamic pressure sensitivity anisotropy of a calcareous shale

Abstract

Optimizing the productivity of nonconventional, low‐permeability “shale” reservoirs requires detailed knowledge of the mechanical properties of such materials. These rocks' elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information. Here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional Duvernay reservoir of Alberta, Canada. The complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 MPa. Wave speed anisotropies of up to 35% are observed at even the highest confining pressures. Furthermore, the stress sensitivity of the wave speeds, and hence moduli, is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure, whereas those along the bedding plane show, unexpectedly, nearly no pressure dependence. These observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images. These results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions, for the concentration of stress within such formations, and for estimation of static engineering moduli from sonic log wave speeds.

Spatiotemporal Variations in Crustal Seismic Anisotropy Surrounding Induced Earthquakes Near Fox Creek, Alberta

“…Further insights could be gained from the spatiotemporal variations in these two components (Figure c). It has been reported that the amount of anisotropy caused by HF in the Duvernay shale can reach up to 30% in rock physical experiments (Ong et al, ). The thickness of the total shale formation is over 90 m (Davis & Karlen, ) and, due to the nearly parallel emergence angle, the range of anisotropy within the HF area is estimated to be 200–400 m, which accounts for 5–10% of the lengths of ray paths.…”

Laboratory experiments simulating poroelastic stress changes associated with depletion and injection in low‐porosity sedimentary rocks

“…The derived effective stress coefficient for ultrasonic velocities (dynamic) in the same specimens [ Ma and Zoback , ] was found to differ from that by the static volumetric strain shown in this study. Recent studies on dynamic and static properties in the same shale specimens also confirmed the fundamental differences between the two physical quantities and their sensitivities to stresses [ Meléndez‐Martínez and Schmitt , ; Ong et al , ]. Although the transmitting waves through the rock also induce strain (by vibration), the amount of dynamic deformation is a few orders smaller than the strain produced by the static loading [cf.…”

Effects of spatial heterogeneity and material anisotropy on the fracture pattern and macroscopic effective toughness of Mancos Shale in Brazilian tests

“… G 13 denoted by the shear modulus along the perpendicular direction of the isotropic plane is assumed to 1.7 GPa [cf. Amadei , ; Debasis and Kumar , ; Ong et al ., ]. The material parameters for stiff (isotropic) and soft (transverse isotropic) layers are summarized in Table .…”

“…Notation: the rotational axis of symmetry is parallel to the x 3 axis, and x 1 ‐ x 2 coordinate system planes are identical [cf. Ong et al ., ].…”

“…To be specific, the presence of platy clay minerals and compliant organic materials in shale significantly influences the spatial microheterogeneity in the layered system [ Amadei , ; Heath et al ., ; Loucks et al ., ; Hart et al ., ]. In addition, the anisotropy of shale is often attributed to many factors including sedimentary bedding layers, irregular features, and mineralogical orientation [e.g., Kaarsberg , ; Cholach and Schmitt , ; Sondergeld and Rai , ; Ong et al ., ]. Lamination with defects or gaps may also lead to a more complex crack path, affecting the measured effective strength and toughness of the shale formation.…”